The present invention relates to digital transmissions, and more precisely to the shaping of one or more streams of information symbols to be transmitted over one or more communication channels.
In the present description, the invention will be described more particularly in its application, nonlimiting, to third generation radio communication networks of the UMTS type (“Universal Mobile Telecommunication System”). A general description of this system is presented in the article: “L'UMTS: la génération des mobiles multimédia” [UMTS: the generation of the multimedia mobile] by P. Blanc et al., L'Echo des Recherches, no. 170, 4th quarter 1997/ 1st quarter 1998, pages 53-68. In this system, the invention finds application within the framework of downlinks, that is to say from the base stations to the terminal equipment, in frequency duplex mode (or FDD, “Frequency Domain Duplex”).
UMTS is a radio communication system using code-division multiple access (CDMA), that is to say the symbols transmitted are multiplied by spreading codes consisting of samples known as “chips” whose rate (3.84 Mchip/s in the case of UMTS) is greater than that of the symbols transmitted. The spreading codes distinguish between various physical channels PhCH which are superimposed on the same transmission resource constituted by carrier frequency. The auto-and cross-correlation properties of the spreading codes enable the receiver to separate the PhCHs and to extract the symbols intended for it. For UMTS in FDD mode on the downlink, a scrambling code is allocated to each base station, and various physical channels used by this base station are distinguished by mutually orthogonal “channelization” codes. For each PhCH, the global spreading code is the product of the “channelization” code and the scrambling code of the base station. The spreading factor (equal to the ratio of the chip rate to the symbol rate) is a power of 2 lying between 4 and 512. This factor is chosen as a function of the bit rate of the symbols to be transmitted on the PhCH.
The various physical channels obey a frame structure illustrated in FIG. 1. The 10 ms frames follow one another on the carrier frequency used by the base station. Each frame is subdivided into 15 time slots of 666 μs. Each slot can carry the superimposed contributions of one or more physical channels, comprising common channels and dedicated channels DPCH (“Dedicated Physical CHannel”). The lower chart of FIG. 1 illustrates the contribution of a downlink DPCH at a time slot in FDD mode, which comprises:                a certain number of pilot symbols PL placed at the end of the slot. Known a priori to the terminal, these symbols PL enable it to acquire the synchronization and to estimate parameters which are useful in demodulating the signal;        a transport format combination indicator TFCI placed at the start of the slot;        a transmit power control TPC to be used by the terminal on the uplink; and        two data fields, denoted DATA1 and DATA2, placed either side of the TPC field.        
The DPCH can thus be seen as amalgamating a physical channel dedicated to control, or DPCCH (“Dedicated Physical Control CHannel”), corresponding to the fields TFCI, TPC and PL, and a physical channel dedicated to the data, or DPDCH (“Dedicated Physical Data CHannel”), corresponding to the fields DATA1 and DATA2.
For one and the same communication, it is possible to establish several DPCHs corresponding to different “channelization” codes, whose spreading factors may be equal or different. This situation is encountered in particular when a DPDCH is insufficient to provide the transmission bit rate required by the application. In what follows, Y will denote the number, equal to or greater than 1, of downlink physical channels used for one and the same communication from a base station.
Moreover, this same communication can use one or more transport channels TrCH. Multiplexed TrCHs are typically used for multimedia transmissions, in which signals of different natures to be transmitted simultaneously require different transport characteristics, in particular as regards protection against transmission errors. On the other hand, certain coders may output, in order to represent a given signal (audio for example), several streams of symbols having different perceptual importances and therefore requiring different degrees of protection. Multiple TrCHs are then used to transport these various symbol streams. In what follows, X will denote the number, equal to or greater than 1, of transport channels used for a given communication on the aforesaid Y physical channels.
For each transport channel i (1≦i≦X), there is defined a transmission time interval TTI composed of Fi consecutive frames, with Fi=1, 2, 4 or 8. Typically, the shorter the delay with which the signal conveyed by the transport channel must be received, the shorter is the TTI used. For example, a TTI of 10 ms (Fi=1) will be used for a telephony application, while a TTI of 80 ms (Fi=8) may be used for a data transmission application.
The multiplexing of the X streams of information symbols emanating from the TrCHs on the Y PhCHs is described in detail in the technical specification 3G TS 25.212, “Multiplexing and channel coding (FDD)”, version 3.0.0 published in October 1999 by the 3GPP (3rd Generation Partnership Project), which can be loaded from ftp://ftp.3gpp.org/Specs/October—99/25_series/.
FIG. 2 diagrammatically illustrates the sending part of a UMTS base station operating in FDD mode. Block 1 denotes the set of sources respectively outputting streams of information symbols ai (1≦i≦X) in relation to the X TrCHs used in a communication.
Block 2 multiplexes the streams ai to form what is referred to as a coded composite transport channel, or CCTrCH, which is subsequently subdivided into one or more physical channels PhCH#j (1≦j≦Y) on which synchronized streams of symbols respectively denoted rj are transmitted.
Block 3 designates the circuits which modulate the streams rj and combine them to form a signal processed by the radio stage 4 before being sent over the air interface. Block 3 caters for the spreading, by the “channelization” codes assigned to the PhCHs, of each of the streams rj, as well as of any additional streams which may be output in respect of other communications supported at the same moment by the base station, the various streams of symbols thus spread being subsequently summed and then multiplied by the scrambling code of the base station. The sequencing and parameterization of blocks 1, 2, 3 is catered for by a control unit 5 in accordance with the parameters defined for the base station and for the relevant communication.
FIG. 3 diagrammatically illustrates the receiving part of a UMTS terminal communicating in FDD mode with a base station according to FIG. 2. Block 7 demodulates the baseband signal restored by the radio stage 6 from the signal picked up by the antenna of the terminal, using the scrambling code of the base station and the Y “channelization” codes assigned to the terminal. For each of the Y physical channels j (1≦j≦Y), block 7 outputs data r′j representing estimates of the symbols of the stream rj formed at base station level.
In the case where the symbols are bits, the estimates r′j are “softbits”, that is to say numerical values whose sign characterizes the estimated bit and whose absolute value represents the likelihood of this estimate.
The Y data streams r′j are delivered to a demultiplexing block 8 which performs the operations inverse to those of the multiplexer 2 of the base station. This block 8 outputs for each transport channel i (1≦i≦X) a stream a′i of estimates (softbits or hardbits) of the symbols of the stream ai. These estimates a′i are delivered to the processing circuit of the TrCH i belonging to the block 9. The sequencing and parameterization of blocks 7, 8, 9 is catered for by a control unit 10 of the terminal.
As is usual in the field of digital radiocommunications, the blocks 1-3,5 of the base station and 7-10 of the terminal can be embodied by programming one or more digital signal processors and/or by using specific logic circuits.
FIGS. 4 and 5 respectively detail the various functional modules of the multiplexing block 2 and demultiplexing block 8 (see the aforesaid specification 3G TS 25.212). In these figures, the references bearing the index i (1≦i≦X) designate the elements referring to TrCH i (blocks 20i and 40i), the references bearing the index j designating the elements referring to PhCH j (1≦j≦Y), and the references with no index referring to the operations performed for each frame at CCTrCH level.
The stream ai to be transmitted on each TRCH i is composed of binary symbols output in the form of successive transport blocks TrBk. The module 21i completes each TrBk by adding thereto a cyclic redundancy code CRC, serving to detect any transmission errors. The TrBk bi are then concatenated and/or segmented by the module 22, so as to form blocks oi of appropriate size for the input of the channel coder 23i.
For each TTI of transport channel i, the channel coder 23i outputs a sequence ci of Ei coded bits denoted ci,m (1≦m≦Ei). Two types of error correcting code may be applied by the module 23i:                a convolutional code of rate ½ or ⅓ and of constraint length K=9;        a turbo code of rate ⅓ for the applications which require the lowest error rates. In this case, the bits ci,3p+q of the output sequence from the coder are systematic bits (copies of the input blocks oi) if q=1, and parity bits if q=2 or 0.        
The bit rate matching modules 24i delete (puncture) or repeat bits of the sequences ci so as to match the bit rate of the TrCHs to the global bit rate allowable on the PhCH or PhCHs given their spreading factors. For each TTI on TrCH i, there is defined, from the information provided by the higher protocol layers, a parameter ΔNiTTI, negative in the case of puncturing and positive in the case of repetition. The sequence gi produced by the module 24i for the TTI is composed of Gi=Ei+ΔNiTTI bits denoted gi,n (1≦n≦Gi). In the case where the module 23i has used a turbo code, the puncturing applied by the module 24i if ΔNiTTI<0 is limited to the parity bits, given the greater importance of the systematic bits to the decoder.
In a given frame, the periods devoted to the various TrCHs of the communication may have fixed positions (before the intra-frame interleaving mentioned below) or variable positions. In the case of fixed positions, it may be necessary to append to the sequence gi, by means of the module 25i, one or more marked symbols which will not be transmitted (the value of the corresponding bit will for example be set to zero instead of ±1 in the output stream rj comprising such a symbol so that the transmission power of the symbol is zero). The DTX (“Discontinuous Transmission”) bits thus marked are denoted “δ”. In the exemplary implementation considered here in a nonlimiting manner, each symbol hi,n of the sequence hi output by the module 25i (0≦n≦Fi.Hi, with Gi≦Fi.Hi) is represented by two bits:                hi,n=(0, gi,n) if n≦Gi;        hi,n=(1, 0) if Gi<n≦Fi.Hi (marked bits “δ”).        
The interleaving module 26i performs a permutation of the sequence hi, with a view to distributing the symbols pertaining to the TTI over the Fi frames which it covers. This interleaving consists in writing the symbols of the sequence hi successively into the rows of a matrix comprising Fi columns, in permuting the columns of the matrix, and in then reading the symbols of the matrix column by column to form the sequence denoted qi. The module 27i then chops the sequence hi into Fi segments of consecutive symbols corresponding to the Fi columns of the interleaving matrix after permutation, and respectively assigns these segments to the Fi frames of the TTI to form a sequence denoted fi for each frame and each TrCH i (1≦i≦X).
In accordance with the specification 3G TS 25.212, the permutation of columns performed by the interleaver 26i is such that the symbol hi,n is found in the frame of rank ni=BR(n−1, Fi) of the TTI, the frames of the TTI being numbered from ni=0 to ni=Fi−1, and BR(x, 2y) being defined as the integer whose representation to the base 2 corresponds to the reading in the reverse direction of the representation to the base 2 on y digits of the remainder from the Euclidean division of x by 2y (for example BR(51, 8)=BR(3, 8)=BR([011]2, 23)=[110]2=6,.
The sequences fi produced for the various TrCHs of the communication (1≦i≦X) are multiplexed, that is to say placed one after the other, by a module 28 forming a sequence s of S symbols for the CCTrCH. In the case where the periods devoted to the various TrCHs of the communication have variable positions, it may be necessary to append to the sequence s, by means of the module 29, one or more marked symbols “δ”. In the exemplary implementation considered here, each symbol wk of the sequence w output by the module 29   (            1      ≤      k      ≤                        ∑                      j            =            1                    Y                ⁢                  U          j                      ,  with   S  ≤            ∑              j        =        1            Y        ⁢          U      j      and Uj equal to the number of bits per frame on the DPDCH of physical channel j, which number depends on the spreading factor allocated to the channel) is represented by two bits:                wk=(0, sk) if k≦S;        wk=(1, 0) if   S  <  k  ≤            ∑              j        =        1            Y        ⁢                  U        j            .              
The module 30 subsequently chops the sequence w into Y segments of Ui, U2, . . . , UY consecutive symbols, and respectively assigns these segments to the Y PhCHs to form a sequence denoted uj for each PhCH j (l≦j≦Y). The interleaving module 31j performs a permutation of the sequence uj with a view to distributing the symbols, within the current frame, over the Y PhCHs employed by the communication. This interleaving consists in writing the symbols of the sequence uj successively to the rows of a matrix comprising thirty columns, in permuting the columns of the matrix, and in then reading the symbols of the matrix column by column to form the sequence, denoted vj, of Uj symbols.
The module 32j for mapping the physical channel finally distributes the successive symbols of the sequence vj into the fields DATA1 and DATA2 of the time slots of the current frame. The module 32j can translate the information bits with values 0 or 1 into signed bits (±1), and assign the value 0 to the marked bits “δ”. It furthermore supplements the stream rj addressed to the block 3 by inserting the appropriate signalling bits into the fields PL, TFCI and TPC of the DPCCH.
The demultiplexing block 8 comprises modules which perform, in the reverse direction, the operations which are dual to those of the modules 20i-32j of the multiplexing block 2. In FIG. 5, the primed references correspond to the estimates of the symbols bearing the same unprimed references in FIG. 4. For the symbols composed of two bits formatted as indicated hereinabove by reason of the marking of the bits “δ”, these estimates (softbits) refer to the least significant bit.
For each 10 ms frame and each PhCH, the module 52j extracts the sequence v′j of Uj softbits pertaining to the DPDCH from the fields DATA1 and DATA2 of the demodulated signal. The deinterleaving module 51j applies the inverse permutation to that of the module 31j to this sequence v′j so as to restore the sequence of softbits u′j. The Y sequences u′j are placed end to end by the multiplexing module 50 so as to form the sequence of softbits w′ which relates to the CCTrCH. In the case where the TrCH have variable positions, the module 49 deletes the last             ∑              j        =        1            Y        ⁢          U      j        -  Ssoftbits of the sequence w′, which correspond to “δ” bits. The softbit sequence s′ produced by the module 49 is chopped by the segmentation module 49 into X subsequences f′i respectively assigned to the TrCHs.
For each TrCH i whose TTI comprises several frames (Fi>1), the module 47i concatenates the subsequences produced in relation to the various frames so as to form the sequence q′i subjected to the inter-frame deinterleaving module 46i. The latter carries out the permutation inverse to that of the module 26i so as to restore the sequence of softbits h′i. In the case where the TrCHs have fixed positions, the module 45i deletes the Fi.Hi-Gi last softbits of the sequence h′i, which correspond to “δ” bits. The sequence of softbits s′ produced by the module 49 is then processed by the bit rate matching module 44i which performs the following operations:                insertion of a null softbit (minimum likelihood) in place of each bit which has been punctured on transmission;        reevaluation of each softbit corresponding to a bit which has been repeated on transmission, so as to sharpen the likelihood thereof.        
The output sequence c′i of the module 44i is decoded by the module 43i so as to correct any transmission errors. The symbols of the decoded blocks o′i output by the module 43i can be softbits, or hardbits if the likelihood measures are no longer required in the subsequent processing. On the basis of these blocks o′i, the module 42i reconstructs the estimated TrBk b′i, and the module 41i verifies the integrity of the CRC so as to validate these TrBk in the output stream a′i relating to TrCH i.
In the UMTS system, in particular in FDD mode, there is provision for the communicating terminals to be furnished with time windows so as to listen to one or more carrier frequencies different from that supporting the communication. This listening procedure allows in particular the terminals equipped with a single radio frequency receiver to perform measurements of radio parameters (module 11 of FIG. 3) with a view to possible automatic transfer (handover):                from a UMTS FDD cell to another UMTS FDD cell using a different carrier;        from a UMTS FDD cell to a UMTS TDD cell (“Time Domain Duplex”); or else        from a UMTS FDD cell to a cell of a second-generation network such as a GSM network.        
During the listening window, which may extend over one or more time slots of 666 μs, the base station interrupts its transmission to the terminal. This interruption is specific to the air interface, and has no impact on the output bit rate of the sources of the block 1 which relate to the various TrCHs. In the course of each 10 ms frame having an inactive period (during which no symbol is transmitted), it is therefore necessary, outside of this inactive period, to increase the transmission bit rate on the Y PhCHs.
These frames are said to use a compressed mode. In order for the quality in terms of binary error rate (BER) or of frame error rate (FER) not to be affected by the interruption of transmission, the base station transmission power is increased in the compressed-mode frames outside of the inactive period.
The interruptions of transmission can take place periodically or on request. In the course of a given frame, the number of time slots covered by the inactive period is a maximum of 7. The illustration of FIG. 1 shows two interruptions of transmission GAP1, GAP2. The interruption GAP1 falls within a single compressed-mode frame T1, while the interruption GAP2 straddles two compressed-mode frames T2, T2′. Interruptions extending over two consecutive frames, such as GAP2, are useful in particular for handovers to the GSM networks requiring a measurement window of 6 ms.
As indicated in the aforesaid specification 3G TS 25.212, an interruption going from slot Nfirst to slot Nlast begins at the TFCI or DATA2 field of the slot Nfirst, and terminates at the field DATA2 of slot Nlast. In both cases, the modules 32j of the multiplexing block 2 generate the inactive period of the compressed-mode frame by placing the information bits in the remaining DATA1 and DATA2 fields.
In compressed mode, two methods A and B can be used to match the bit rate of the PhCHs to that of the TrCHs.
Method A consists in an additional puncturing (relative to that which may be applied by the bit rate matching module 24i), serving to create the interruption of transmission in each frame concerned.
Method B consists in dividing by 2 the Y spreading factors employed in the compressed-mode frames. A limitation of this method B is that it requires the availability of spreading codes of half factor, thereby penalizing the code resources in the cell.
Method A poses a problem when the communication uses at least one TrCH whose transmission time interval covers several frames (Fi>1): if one of these frames is in compressed mode, then the additional puncturing must be carried out specifically in this frame, this being tricky given the interleaving applied by the modules 26i and the shifts generated in the sequence of symbols by the bit rate matching module 24i. An additional constraint in the case where a turbo code is used for the channel coding on a TrCH is that it is not desirable to puncture systematic bits.
Accordingly, the compressed mode according to method A requires a priori fairly significant modifications to the multiplexing and demultiplexing suite according to FIGS. 4 and 5, and hence an increase in the complexity of the base stations and terminals, which must naturally be compatible both with the compressed mode and with the noncompressed mode.
An aim of the present invention is to limit the impact of these problems in the systems using a processing suite of the kind described above.